The present invention is directed to a lamp ballast circuit which includes a gate driver circuit for driving MOS gated devices and, more particularly, to a lamp ballast circuit which is protected against the removal of or failure of the lamp.
Electronic ballast circuits for driving fluorescent lamps or other gas discharge illumination devices are coming into widespread use because of the availability of power MOSFET switching devices to replace the previously used bipolar transistor devices. Typically, the electronic ballast circuit uses two power MOSFET switches in a totem pole (half-bridge) arrangement which includes one or more L-C series resonance circuits and in which the lamp or lamps are connected across one of the reactances of the L-C circuit. The power MOSFET switches are driven to conduct alternately by inputs from the secondary windings of a current transformer whose primary winding conducts the current of the lamp circuits. The primary winding current alternates at the resonance frequency of the resonant circuit.
Recently, integrated circuit MOS gate driver devices have been introduced in place of the current transformers. These integrated circuit devices drive the power MOSFETs or IGBTs of an inverter circuit in the ballast circuit from logic level ground referenced inputs and provide a self-oscillating function which is particularly suited for use in electronic lamp ballast circuits. The integrated circuit devices significantly save cost, weight and space when compared to driver circuits employing current transformers.
The MOS gate driver circuits have the drawback, however, that when the lamps are removed or are broken and create an open circuit, the power MOSFETs continue to switch in a hard switching mode with high power dissipation and fail due to overtemperature. It is therefore greatly desirable that the ballast circuit be able to shut down when there is an open circuit condition.